The present invention relates generally to flow cytometers. More particularly, the present invention relates to optical detection systems for flow cytometer systems.
Flow cytometry is a technique that is used to determine certain physical and chemical properties of microscopic biological particles by sensing certain optical properties of the particles. Flow cytometry is currently used in a wide variety of applications including hematology, immunology, genetics, food science, pharmacology, microbiology, parasitology and oncology.
In flow cytometry, the microscopic biological particles of a sample fluid are arranged in single file in a core stream, typically using hydrodynamic focussing. The particles are then individually interrogated by an optical detection system. The optical detection system provides a light beam, which is scattered by each particle to produce a scatter profile. The scatter profile is analyzed by measuring the light intensity at both small and larger scatter angles. Certain physical and/or chemical properties of each particle can then be determined from the scatter profile.
Conventional cytometer systems use a single light source such as a laser to interrogate each particle. The light beam is often focussed to an elongated shape that covers the uncertainty in particle position due to misalignment and variations in the width of the core stream. A limitation of using a single light source is that the particle position and variations in the width of the core stream cannot be directly detected. Misaligmnents in particle position and variations in the width of the core stream can be indicators of improper core formation. Because there may be no direct way of monitoring the characteristics of the core stream, improper core formation may go undetected.
This limitation may be further compounded because the single laser source configuration often does not provide a constant illumination intensity across the flow channel. As such, particles that pass more toward the edge of the core stream may not be as illuminated as particles that pass near the center. As a result, the sensitivity and accuracy of the system may vary depending on the lateral position of the particle through the focused elongated shape beam. Since there may be no easy way of detecting the lateral position of each particle, the variations in sensitivity and accuracy may go undetected.
Another limitation of using a single light source is that the velocity of each particle cannot be directly determined. Particle velocity is often an important parameter in estimating the particle size from light scatter signals. In conventional flow cytometry systems, the velocity of each particle is extrapolated from the pump flow rates. Accordingly, to accurately gauge the velocity of each particle, the pumps must be very precise, the tolerance of the cytometer flow chambers must be tightly controlled, no fluid failures such as leaks can occur, and no obstructions such as microbubbles can be introduced to disturb the flow or core formation. Satisfying these constraints can add significant complexity and cost to the flow cytometer system.
The present invention overcomes many of the disadvantages of the prior art by providing an optical detection system that uses two or more light sources positioned laterally at different distances from the central axis of the flow stream for providing light through different parts of the flow stream. By using two or more light sources, the particle position can be detected, and the alignment and width of the core stream can be monitored and controlled. In addition, the velocity and size of the particles can be more accurately determined than in single light source systems.
In one illustrative embodiment of the present invention, a linear array of first light sources that extend along a first light source axis are provided. The first light source axis is rotated relative to the central axis of the flow stream. A lens is provided adjacent each light source to focus the light at the particles in the core stream. A first set of light detectors are placed in-line with each of the first light sources.
Such an arrangement can be used to determine, for example, the lateral position of each of the particles in the core stream, and the alignment and width of the core stream within the overall flow stream. If the core stream of particles has an improper width or is not in proper alignment, a controller can be used to adjust the fluid velocity of the sample fluid or one of the supporting fluids to adjust the width of the core stream or bring the core stream into alignment. The first set of light detectors may also be used to detect the time-of-flight or velocity of each particle, the size of each particle, as well as the number of particles that flow by the detector.
A second set of the light sources may also be provided along a second light source axis. A lens may be provided adjacent each light source to focus the light at the particles in the core stream. A second set of light detectors may then be placed on either side of the in-line position of each light source for measuring the small angle scattering (SALS) produced by selected particles in the flow stream. The second set of light sources may also be used in conjunction with the first set of light sources to determine the time-of-flight or velocity of the particles in the flow stream. By knowing the velocity of the particles, small variations in the flow rate caused by the fluid driver can be minimized or removed by the controller.
A third set of light sources may be provided along a third light source axis. A lens may be provided adjacent each light source to provide collimated light to the flow stream. Annular light detectors may then be placed opposite the light sources for measuring the forward angle scattering (FALS) produced by the selected particles in the flow stream.
The optical detection system of the present invention may be used in conjunction with a portable cytometer system for detecting, for example, neutrophils and/or lymphocytes white blood cells in a blood sample. By examining the scatter profile of each of the particles, the portable cytometer may identify and count the neutrophils and lymphocytes in the blood sample, and provide a clear infection warning with differentiation between viral and bacterial causes.